Plant Answer Line Question

I have some insects on my bay laurel, which we use for seasoning. Someone told me to spray it with horticultural oil. I wondered, though, if it would still be safe to use the leaves in cooking. Is horticultural oil petroleum based?

Answer:

As you suspected, horticultural oil is petroleum-based. U.C.Davis provides information on Integrated Pest Management for the plant I am assuming you mean, the usual source of culinary bay leaves, sometimes called sweetbay, or Laurus nobilis. (Just to be clear, I don't think you are referring to the plant known as California bay laurel, Umbellularia californica, which has leaves that are aromatic and reportedly edible, but not commonly used for seasoning.)
They also note that horticultural oils are
"...specially refined petroleum products, often
called narrow-range, superior, or supreme oils. Some botanical
(plant-derived) oils are also available."

When referring to plant-derived oils, I believe they are referring to
Neem oil, as described in this information from U.C. Davis Integrated Pest Management. Additional information from the Cornell University Resource Guide for Organic Inesct and Disease Management about Neem and human health is excerpted here:

Studies of azadirachtin mutagenicity and acute
toxicity have shown that it likely does not pose a significant risk to
human health. However, some people have exhibited skin and mucous
membrane irritation from neem seed dust (Weinzierl and Henn 1991). Note
that most studies have been done on azadirachtin, and may not show the
effects of a whole neem product. Neem is used in some commercial human
hygiene products.

Another long excerpt on horticultural oil from Colorado State University Extension suggests the following:

Essentially all commercially available horticultural oils [...] are refined petroleum products also known
as mineral oils. Impurities in the oil that are associated with plant
injury, such as aromatic compounds and compounds containing sulfur,
nitrogen or oxygen, are removed. Filtration, distillation and dewaxing
complete the production of the finished base oil. Final formulations of
horticultural oils are normally combined with an emulsifying agent that
allows the oil to mix with water. This mixture usually is used at about a
2 percent dilution.

Vegetable oils also can be used as insecticides, although the type of oil
can greatly affect its activity. Cottonseed oil is generally considered
the most insecticidal of the vegetable oils. Soybean oil, the most
commonly available vegetable oil used in cooking, has often provided fair
to good control of some insects and mites.

Extracts from seeds of the neem tree, Azadirachta indica, have recently
attracted attention as a source of pest management products. Several
neem-derived insecticides have been developed. A number of compounds
found in neem seeds, notably azadirachtin, have proven useful as
insecticides. However, the oil fraction of neem seed extracts, which is
mostly free of azadirachtin and related terpenoid compounds, also has
demonstrated effects as a fungicide and insecticide. At least one product
currently on the market, TrilogyR, consists of a largely
azadirachtin-free oil fraction of neem seed extracts. It is formulated
with an emulsifier and mixed with water at a concentration similar to
horticultural oils (0.5 to 2.0 percent). Many over-the-counter products
sold in nurseries that mention neem contain the oils of neem seed
extracts.

If you know the insect on your bay tree, Peaceful Valley Farm Supply is one commercial supplier which carries less toxic, of not organic, products to control specific pests. That does not necessarily mean they are safe enough to spray on leaves which will be harvested for cooking.

Plant Answer Line Question

I think my rose leaves are being devoured by rose sawfly, and I was wondering if spraying 'Rose Defense' on them would help.

Answer:

Rose Defense is a Scott's product that contains Neem (as well as other ingredients). There is some evidence that Neem is effective against sawfly larvae. As with any pesticide, you should follow the directions on the package carefully (and note that this product may be harmful to humans, domestic animals, bees, and the environment, depending on the route of exposure).

You might want to start out with the least toxic approach first, that is, handpicking and spraying with water. Once larvae are knocked off the roses, they will not climb up again. If this doesn't seem to be helping, then you could choose a Neem-based spray or insecticidal soap, keeping in mind that the Neem product is toxic to bees, and should not be applied when bees are active.

According to University of Minnesota Extension, sawflies are best controlled when young. You can simply pick them off by hand or dislodge them with a stick or a stream of water. If using water be sure to spray early enough in the day for the foliage to dry by sunset. This will prevent favorable conditions for fungal development. Horticultural oil, insecticidal soap and azadirachtin (sometimes called neem), are among the less toxic insecticides to treat young sawflies. Azadirachtin is slower acting. Bacillus thuringiensis (Bt) is effective on young lepidoptera caterpillars but NOT on larval sawflies.

Cornell University's Resource Guide for Organic Insect and Disease Management (formerly available on the Cornell website) also discusses the uses of Neem. Here is an excerpt:
"Neem products are generally sold as emulsifiable concentrates. Neem oil soap is sold as a water-soluble liquid concentrate. While Copping (2001) reports no known incompatibilities with other crop protection agents, phytotoxicity may be a problem when combining neem oil or soap products. Read labels for specific application guidelines including determination of re-entry interval and pre-harvest interval. Range of efficacy will depend on the susceptibility of species in question and environmental conditions at time of application. However these are points to follow:
Make multiple applications. Frequent applications are more effective than single sprays because neem does not persist well on plant surfaces. Like most other botanically derived materials, it can be rapidly broken down by sunlight and washed away by rain (Thacker 2002).
Use against immature insects. Azadirachtin-based insecticides act on immature stages of insects more effectively than on eggs or adults. To reduce a build up of populations it is important to make treatments to crops targeting insects in an early stage of their life cycle. For instance, neem would likely have little effect on an infestation of striped cucumber beetle adults; however if applied to potato plants early in the season, it has been shown to greatly reduce larval activity of Colorado potato beetle.
Begin applications before pest levels are high. Antifeedant and egg-laying repellant effects show best results in low to moderate pest populations.
Neem is reported to work best under warm temperature conditions (Schmutterer 1990)."

There are quite a few different species of sawfly, and I would guess that the rose sawfly is so named because rose bushes are its primary feeding ground. If you aren't sure what is eating your roses, you may want to take samples of the affected leaves to your local county extension agent before you begin to treat the problem. You may find the images on the self-described Buggiest Rose Website helpful in comparing with the leaf damage you are seeing.